How many calories can I consume?

I'm 45 yrs old, 5"6 and 150lbs. I would like to lose at least 15-20 lbs. I've always been thin about 120lbs but in the last 10 yrs have had 3 kids and of course gotten older and the pounds have crept up. I try to walk/run at least 2 miles 4 times a week. What is the minimum amount of calories I can ingest and still lose weight?